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Motion Control
Special Edition
In a full electrical revolution this
results in a plateau around the zero-
crossing area of the sine wave when
the sign (direction) of the current
changes. The impact of this plateau
is a small period with zero current in
the motor windings, meaning there
is no torque at all. This leads to
wobbling and vibrations, especially
at lower speeds.
In contrast to a constant off-time
chopper, Trinamic's SpreadCycle™
PWM chopper mode applies a
hysteresis functionthat automatically
uses a fitting relation between slow
and fast decay periods. The average
current reflects the configured
nominal current. There is no plateau
in the zero-crossing region of the
sine wave. This reduces current and
torque ripple and a true sine wave
form is approached, resulting in a
much smoother motor operation
compared to a constant off-time
PWM chopper. This is especially
important at standstill and slow to
moderate speeds.
How To Totally Silence
Stepper Motors
Although microstepping reduces a
large part of the vibration caused by
low step resolutions, high microstep
resolutions make it easier to perceive
other sources of vibration. Advanced
current-controlled PWM chopper
modes like Trinamic's SpreadCycle™
algorithm, which is implemented
in hardware, reduce vibration and
wobbling to a large extent. This
is sufficient for many standard
applications and also ideally suited
for higher-speed applications.
But even with current-controlled
chopper modes like SpreadCycle
there is still a little bit of audible noise
and vibration due to unsynchronized
motor coils, regulation noise
of a few millivolts at the sense
resistors, and PWM jitter. This
noise and vibration can be critical
for high-end applications, slow- to
moderate-speed applications, and
any applications where noise is
unacceptable. It was intolerable
for the Dereneville DTT-01-S linear
tracking tonearm, because the noise
coming from the microstepping drive
and hybrid stepper is superimposed
on the audio signal, especially within
the plain grooves at the transition
between individual tracks.
Trinamic's StealthChop™ algorithm
[4], also implemented in hardware,
ultimately silences stepper motors.
But what is StealthChop actually
doing to a stepper motor, and why
doesn't it generate additional noise
and vibrations? StealthChop follows
a different approach compared
to current-based chopper modes
like SpreadCycle: it is a voltage
chopper-based technology that's
responsible for the noiseless
and smooth movement of the
Dereneville DTT-01-S tone arm
and stylus. Combined with closed-
loop tracking angle regulation and
precision laser optics, this results in
a maximum tracking angle error of
headshell plus stylus of <0.05°. A
good conventional pivoted tonearm
has a typical tracking angle error
of <2°-3°, and also suffers from
skating forces and mechanical wear
of the groove.
The TMC5130A-TA – a small, smart
stepper motor driver and controller
IC that includes StealthChop mode
– was the ultimate solution for this
remarkable analog record player. In
addition to StealthChop, Trinamic
has improved voltage mode
operation and combined it with
current control. To minimize current
fluctuation, the TMC5130A-TA chip's
driver regulates voltage modulation
based on the current feedback. This
allows the system to self-adjust to
the motor's parameters and the
operating voltage.
Small oscillations caused by the
regulation algorithms of direct
current control loops are eliminated.
As SpreadCycle and other current-
regulated chopper principles always
react to the coil current measurement
on a cycle-by-cycle basis, a few
millivolts of noise – which are always
present in complex systems – as well
as electric and magnetic coupling
between both coils within the motor,
lead to small variations of the resulting
motor currents and thus influence the
chopper. Figures 10 and 11 compare
voltage-controlled StealthChop with
current-controlled SpreadCycle. The
zero-crossing behavior of StealthChop
is perfect: when the signs of the
current value change from plus to
minus or vice versa, there is no
plateau, but a straight crossing of the
zero-current level, since the current
Figure 8:
Zero-crossing plateau with classic off-
time chopper modes
Figure 9:
SpreadCycle hysteresis chopper with clean
zero crossing
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